Generally, the recent printer is required to be able to handle both cut sheets and continuous fanfold papers. Since the cut-sheet feed system for feeding cut sheets and the fanfold paper feed system for feeding continuous fanfold papers are different from each other, the paper handling mode of the printer must be changed selectively between a mode using the cut-sheet feed system and a mode using the fanfold paper feed system. The cut-sheet feed system presses a cut sheet against a platen with a pinch roller and rotates the platen to feed the cut sheet by the frictional traction of the platen. The fanfold paper feed system has tractor wheels which engage perforations formed in the edge of a fanfold paper and rotate to feed the fanfold paper by traction. It sometimes becomes necessary during printing operation for printing on a fanfold paper to interrupt printing on a fanfold paper and to print temporarily on a cut sheet. In such a case, the fanfold paper is not removed from the printer and is retracted from the platen with the tractor wheels engaging the perforations of the fanfold paper to a predetermined standby postion, a cut sheet is pressed against the platen with the pinch roller for printing, and then the fanfold paper is brought again into engagement with the platen to restart printing on the same fanfold paper after the printing operation for printing on the cut sheet has been completed. If power is transmitted continuously to the tractor wheels while the tractors are held at a standby position, the fanfold paper held standby is moved to cause jamming or the displacement of the present print line. Accordingly, it has been necessary to transmit power to the tractor wheels for printing on a fanfold paper and to disconnect the tractor wheels from the driving source during printing on a cut sheet.
FIGS. 17 and 18 illustrate a conventional paper feed mode change-over mechanism. A platen 1 is mounted fixedly on a platen shaft 2 supported substantially in a horizontal position for rotation on a frame, not shown. A print head 3 is provided for reciprocation along the platen 1. A driven gear 4 fixed to one end of the platen shaft 2 is in continuous engagement with a driving gear 5 which is driven by a driving device, not shown. A tractor unit 6 includes a tractor shaft 7 and a tractor gear 8 fixed to one end of the tractor shaft 7 so as to be located in a plane including the driving gear 4 and separated from the driven gear 4. A swing arm 10 has one end rotatably supporting an idle gear 9 and the other end provided with a slot 11 having two parallel sides. A rotary shaft 12 extended in parallel to the platen 1 has one end fixedly connected to a lever 13 and the other end having a shaped portion 14 having two flat surfaces and pressed in the slot 11 of the swing arm 10 supporting the idle gear 9 to connect fixedly the rotary shaft 12 and the swing arm 10. When the rotary shaft 12 is turned by the lever 13 in one direction or the other, the swing arm 10 is caused to swing to a position where the idle gear 9 engage both the driven gear 4 and the tractor gear 8 or to a position where the idle gear 9 is disengaged from both the driven gear 4 and the tractor gear 8. Thus, the driven gear 4, the tractor gear 8, and the idle gear 9 constitute an interlocking gear train A.
The printer equipped with this paper feed mode changeover mechanism is able to use both cut sheets and fanfold papers. In printing on a cut sheet, the lever 13 is turned in a counterclockwise direction as viewed in FIG. 17. Thereby the rotary shaft 12 and the swing arm 10 fixed to the rotary shaft 12 are caused to turn in a counterclockwise direction through a predetermined angle to disengage the idle gear 9 from both the driven gear 4 and the tractor gear 8 so that power transmission to the tractor unit 6 is interrupted. Then, the cut sheet is inserted between the platen 1 and the pinch roller (not shown) and is fed frictionally by the platen 1 as a platen 1 is rotated, while the print head 3 reciprocates along the platen 1 for printing.
In printing on a fanfold paper, the lever 13 is turned in a clockwise direction as viewed in FIG. 17 and is held at a fixed position. Consequently, the rotary shaft 12 and the swing arm 10 fixed to the rotary shaft 12 are turned in a clockwise direction through a predetermined angle to bring the idle gear 9 into engagement with both the driven gear 4 and the tractor gear 8, and thereby power is transmitted to the tractor unit 6. In this state, the pinch roller is separated from the platen 1 and a tractor wheel 15 of the tractor unit 6 engages the perforations formed in the edge of the fanfold paper. The fanfold paper slips at a small slip rate relative to the surface of the platen 1 as the same is fed by the tractor unit 6 at the feed rate of the tractor unit 6, while the print head 3 reciprocates along the platen for printing.
This known paper feed mode changeover mechanism is capable of connecting the tractor unit 6 to and disconnecting the same from the driving unit through a simple operation. However, the known paper feed mode changeover mechanism has problems. Although the paper feed mode changeover mechanism is designed so that the idle gear 9 is able to engage properly both the driven gear 4 and the tractor gear 8, the paper feed mode changeover mechanism must be constructed in a high accuracy to make the idle gear 9, which moves along a circular path, engage both the driven gear 4 and the tractor gear 8 exactly . It often occurs in the interlocking gear train A that although able to engage one of the two gears 4 and 8 exactly, the idle gear 9 is unable to engage the other gear exactly. Furthermore, it is possible that the gear teeth are damaged or the shafts are distorted when the idle gear 9 is caused to engage the gears 4 and 8 forcibly with the tooth crests thereof in contact with those of the gears 4 and 8. Even if the gears are brought into engagement smoothly, the gears are not necessarily engaged properly due to inevitable errors in size of the parts that is, it is possible that the depth of engagement is excessively large or excessively small. If the printer is operated with the gears 4, 8, and 9 of the interlocking gear train A engaged in an excessively large depth of engagement, the gear teeth are abraded rapidly, parts of the printer are worn rapidly, and prints are deformed because the gears in the interlocking gear train A are unable to function smoothly. On the other hand, if the gears are engaged in an excessively small depth of engagement, the excessive backlash causes free movement of the gears relative to each other entailing the deformation of prints. Furthermore, since the elongated swing arm 10 supporting the idle gear 9 on the extremity thereof is unable to hold the idle gear 9 firmly in engagement with the driven gear 4 and the tractor gear 8, the idle gear 9 is liable to be disengaged easily from the driven gear 4 and the tractor gear 8 when an excessive torque is applied thereto in reversing the paper feed direction.
These troubles can be obviated by forming the parts in a higher accuracy to locate the idle gear 9 at an optimum position and by controlling the phases of the gears 4, 8, and 9 so that the tooth crests of the mating gears will not come into abutment with each other in engaging the gears 4, 8, and 9, or by employing a mechanism capable of firmly holding the idle gear 9 in engagement with the driven gear 4 and the tractor gear 8 and capable of fine positional adjustment of the idle gear 9. However, such measures require a sophisticated mechanism and entail difficulties in design, manufacture, and maintenance, deterioration of the accessibility of the printer, and increase in the manufacturing cost. Thus, these measures are not practically applicable.